WASHINGTON/SAN FRANCISCO • US legislation may be needed to require social media companies to disclose more about how their platforms are used for political advertising, a senior Democratic lawmaker has said, after new signs of Russian efforts to influence the 2016 election.

Facebook said last Wednesday that an operation likely based in Russia had placed thousands of US ads with polarising views on topics such as immigration, race and gay rights on the site during a two-year period to May this year.

The world's largest social network was already under fire for how little transparency it provides about digital political ads.

Mr Mark Warner, the top Democrat on the Senate Intelligence Committee, said Congress may need to update laws in order to make them consistent with rules governing television advertising.

"An American can still figure out what the content is being used in TV advertising. You can go look at the ad," Mr Warner said at a security conference last Thursday.

"But in social media, there is no such requirement. So, you know, we may need a legislative solution," he said.

The Intelligence Committee's chairman, Mr Richard Burr, however, rejected the idea of moving legislation through his panel and added that foreign interference in US elections is already illegal.

US intelligence agencies say there was an extensive Russian cyber-influence operation during the 2016 election campaign aimed at helping Mr Donald Trump, a Republican, defeat Democrat Hillary Clinton.

Watchdog group Common Cause last Thursday filed a complaint with the US Justice Department and the Federal Election Commission alleging that unknown foreign nationals made expenditures during the election in violation of American election law.

Mr Warner said what Facebook disclosed was "the tip of the iceberg" and that Twitter and other tech firms should also examine the issue. He said representatives of Twitter will be meeting the Senate panel soon. Twitter declined to comment.

REUTERS, BLOOMBERG